Aluminum paste that has been conventionally adopted for aluminum back electrodes of a photovoltaic device such as a solar battery has an advantage that the electrodes can be readily formed by screen printing or the like. Furthermore, the aluminum paste has another advantage that a so-called back surface field (BSF) layer, which improves the efficiency of collecting majority carriers, can be readily formed by diffusing aluminum into the silicon substrate and thereby readily forming by heat treatment a p+ layer in which p-type impurities are diffused in high concentration, and generating a barrier electric field inside the photovoltaic device against minority carriers.
The solar battery manufacturing process that is currently in wide use is briefly explained below.
(101) First, a p-type silicon substrate from which a damaged layer formed at the time of slicing is prepared, and a fine uneven structure called texture is created on the surface of the p-type silicon substrate by etching the p-type silicon substrate with an alkaline solution.(102) After diffusing phosphorus oxychloride (POCl3), phosphoric acid or the like into the light receiving surface of the p-type silicon substrate and thereby forming an n-type impurity diffusion layer, a film predominantly containing glass is removed from the surface, and the n-type impurity diffusion layer is also removed from the end surfaces and the back surface.(103) An antireflective film is formed on the light receiving surface of the p-type silicon substrate on which the n-type impurity diffusion layer is formed.(104) A paste containing silver and glass is applied partially on the opposite surface (back surface) of the p-type silicon substrate with respect to the light receiving surface, in the shape of a collective back electrode.(105) A paste containing aluminum and glass for formation of the aluminum back electrodes is applied onto the area of the back surface of the p-type silicon substrate where no collective back electrode is arranged.(106) The paste containing silver and glass is applied in the shape of light receiving surface electrodes onto the light receiving surface of the p-type silicon substrate.(107) The collective back electrode, the aluminum back electrodes, and the light receiving surface electrodes are obtained by firing at a certain temperature in the air, and aluminum is diffused from the aluminum back electrodes into the silicon substrate to form a BSF layer. At the same time, the so-called fire through process occurs, by which silver from the light receiving surface electrodes penetrates through the antireflective film and establishes an electrical connection with the n-type impurity diffusion layer, and thereby a solar cell is completed.
The above process allows all the electrodes to be formed in a single firing process, and thus this process is most widely used at present.
In the meantime, concerns are rising that silicon materials could become in short because of prospective rapid growth of silicon solar batteries. As a measure against this, the thickness of the silicon substrate may be reduced from the conventional order of 200 micrometers to a smaller thickness. Then, the silicon materials can be efficiently used, and the manufacturing cost of solar batteries can be reduced, while the production volume can be increased.
However, if the thickness of the silicon substrate is to be reduced, during the firing operation for electrode formation in the above process, the difference between the linear expansion coefficients of aluminum and silicon incurs warpage at the time of cooling in a subsequent stage in the firing operation. This significantly increases the breakage rate of solar cells during the manufacturing process.
The aforementioned warpage that occurs in the solar cell could be reduced by thinning the applied aluminum paste. However, if the thickness of the aluminum paste is too small, the thickness of the BSF layer that is formed by firing also becomes small, which prevents the solar cell from maintaining its characteristics.
In addition, as the thickness of the silicon substrate is reduced, multiple reflection should be caused inside the silicon substrate and the optical path should be extended in order to fully use long-wavelength light that has a small absorption coefficient for electricity generation. However, because the aluminum back electrodes formed of aluminum has a low reflectance with respect to the long-wavelength light, the utilization efficiency decreases and the amount of current generation is lessened.
For this reason, a method has been conceived with which, in place of the thinned BSF layer that is formed by heating the aluminum paste, a back-surface passivation film that inactivates defects on the back surface of the silicon substrate is incorporated so that the excellent characteristics of the solar cell can be maintained while warpage of the silicon substrate can be avoided. More specifically, a method of forming a back-surface passivation film such as a silicon nitride or silicon oxide film on the back surface of the silicon substrate, forming contact holes in this back-surface passivation film, and electrically connecting the aluminum back electrodes to the silicon substrate has been disclosed (see, for example, Patent Document 1).
However, the method according to Patent Document 1 requires a step of preparing openings in the back-surface passivation film with a laser to form contact holes and a step of aligning the aluminum back electrodes precisely to the laser-processed openings. In addition, because the firing operation has to be conducted several times, the number of processing steps significantly increases, in comparison with the conventional method that has been in wide use. The typical method of manufacturing a solar battery by preparing openings in the back-surface passivation film with a laser and electrically connecting the silicon substrate to the back electrodes is explained below. The steps (111) to (113) for forming an n-type impurity diffusion layer and an antireflective film on the light receiving surface of the p-type silicon substrate are the same as (101) to (103) of the conventional method.
(111) First, a p-type silicon substrate from which a damaged layer formed at the time of slicing is removed is prepared, and the p-type silicon substrate is etched with an alkaline solution to form a fine uneven structure called texture on the surface of the p-type silicon substrate.(112) After phosphorus oxychloride (POCl3), phosphoric acid, or the like is diffused in the light receiving surface of the p-type silicon substrate to form an n-type impurity diffusion layer, a film predominantly containing glass is removed from the surface, and the n-type impurity diffusion layer is also removed from the edge surfaces and the back surface.(113) An antireflective film is formed on the light receiving surface of the p-type silicon substrate on which the n-type impurity diffusion layer is formed.(114) A back-surface passivation film is formed on the back surface of the p-type silicon substrate by use of silicon nitride or a silicon oxide film.(115) Contact holes are formed in the back-surface passivation film with a laser.(116) A paste containing aluminum, glass, and the like is applied on the contact holes formed in the back-surface passivation film to form aluminum back electrodes.(117) firing is conducted in the air at a certain temperature to form aluminum back electrodes and also a BSF layer. The method of setting the level of oxygen lower than or equal to 100 ppm at this time and thereby reducing the electrical resistance to the collective back electrode that is to be formed at the next step is disclosed in Patent Document 1.(118) A paste containing silver, glass, and the like is applied onto the aluminum back electrodes to form a collective back electrode. Moreover, the paste containing silver, glass, and the like is applied onto the light receiving surface of the p-type silicon substrate to form light receiving surface electrodes.(119) firing is conducted in the air at a certain temperature to form the collective back electrode and the light receiving surface electrodes, the silver in the light receiving surface electrodes penetrates through the antireflective film to establish an electrical connection with the n-type impurity diffusion layer, and thereby the solar cell is completed.
With the above method, an efficient solar cell with small warpage can be produced by forming the back-surface passivation film and the BSF layer. However, the number of steps is significantly increased, in comparison with the aforementioned conventional method.
Furthermore, Non-patent Document 1 discloses, as a passivated emitter and rear cell (PERC) structure, a solar battery produced by a method, with which a back-surface passivation film is formed by use of the silicon oxide film and openings are created in this back-surface passivation film. As described above, when a silicon oxide film is adopted for the back-surface passivation film, steps of preparing a resist pattern on the back-surface passivation film by photolithography and forming openings in the silicon oxide film with hydrofluoric acid are required. With this method also, the number of steps is significantly increased, and thus volume production of solar cells cannot be efficiently performed.
Moreover, a method of forming a dot pattern of an aluminum paste on the back-surface passivation film by screen printing or the like and electrically connecting it with the silicon substrate by the so-called fire-through process has been disclosed, for example, in Patent Document 1. According to the embodiments of Patent Document 1, after a back-surface passivation film is formed, a dot pattern of a paste containing aluminum, glass, and the like is formed and fired on the back-surface passivation film, and thereby a BSF layer is formed. Then, a collective back electrode and light receiving surface electrodes are formed of the paste containing silver, glass, and the like and subjected to the firing to complete the solar cell.    Patent Document 1: Japanese Patent Application Laid-open No. 2007-299844    Non-patent Document 1: Appl. Phys. Lett. vol. 55 (1989), p. 1363-1365